Surface acoustic wave (SAW) devices are much used today in electronic communication, especially SAW devices arranged to provide filtering functions. Filters formed from SAW devices are particularly useful in connection with paging apparatus, communications links, portable radio and telephones. Such radio telephones operate typically in the 500-1500 MHz range and higher.
Filter characteristics that are of particular interest to the radio designer are: (a) passband bandwidth, i.e., a range of frequencies wherein the filter passes a signal with acceptable loss, (b) passband attenuation, i.e., how little passband loss is incurred, (c) transition bandwidth, i.e., a range of frequencies separating pass- and stop-bands and (d) stopband attenuation, i.e., attenuation outside pass- and transition-bands where no signal is desired to be transmitted through the filter. Stopbands are frequency regions wherein the filter provides very high attenuation. The transition band is the frequency region in which the attenuation rapidly increases from a low value (little attenuation) at the passband corner frequency to a high value (large attenuation) in the stopband. The transition band is also referred to in the art as the "skirt" of the filter transfer characteristic, e.g., the fall-off region on either side of the passband on a plot of filter attenuation versus frequency.
It is a feature of prior art SAW filters that the pass- and transition-bandwidths are related. The choice and cut of piezoelectric substrate material from which the SAW filter is constructed, the number of electrodes and electrode shape, spacing and location influence SAW filter characteristics. Design modifications causing increased passband bandwidth together with optimal insertion loss generally also increase transition bandwidth. Conversely, those choices providing small transition bandwidths also yield narrow passband bandwidths. This effectively precludes SAW filter designs from simultaneously providing relatively wide passband bandwidth together with very narrow transition bandwidths, i.e., steep skirts on the filter transfer characteristic.
An advantage of the present invention is that it overcomes these limitations of the prior art so that greater passband bandwidth is obtained while preserving a narrow transition-bandwidth (steep skirt) on at least one side of the passband. Such SAW filters are advantageously used to improve the properties of radios.